We are able to balance each one of the bus cards for our 6K(A, B, and C bus) separately. My guess is that there's a card in the center section that allows this for the 4K. On ours the cards are labels 82E256. The pot you want to look for is labeled VR1. A tech could shed more light on this, but that's what we typically do to balance all the buses on the 6K. An extender card would be very helpful on this, but not absolutely necessary.
